fix(graperank): run reachability probe on an isolated thread pool

The probe does blocking DNS + TCP connect, and dead-domain DNS lookups hang well
past the connect timeout. On the shared Dispatchers.IO those hanging lookups
starved the crawl's own IO: an A/B at hop-3 showed probe-on 981s vs probe-off
517s, the entire +464s landing on the finishing drain (rounds were identical).
Coverage was unchanged (91.84% vs 91.74%), so the probe classification is correct
— it was purely IO contention.

Give the probe its own fixed daemon pool (128 threads) so its blocking work can
never touch the crawl's IO, and align the culler's concurrency to it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MSW59hJtP4Yn8fnRUxc7F5

cli/src/main/kotlin/com/vitorpamplona/amethyst/cli/commands/GrapeRankCommand.kt

@@ -54,6 +54,7 @@ import com.vitorpamplona.quartz.nip85TrustedAssertions.users.ContactCardEvent
 import com.vitorpamplona.quartz.nip85TrustedAssertions.users.tags.RankTag
 import kotlinx.coroutines.CancellationException
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.asCoroutineDispatcher
 import kotlinx.coroutines.async
 import kotlinx.coroutines.awaitAll
 import kotlinx.coroutines.coroutineScope
@@ -61,6 +62,7 @@ import kotlinx.coroutines.withContext
 import java.net.InetSocketAddress
 import java.net.Socket
 import java.net.URI
+import java.util.concurrent.Executors
 import kotlin.math.roundToInt
 
 /**
@@ -129,6 +131,15 @@ object GrapeRankCommand {
 
     private const val PROBE_TIMEOUT_MS = 2000
 
+    // The probe does BLOCKING DNS + TCP connect, and dead-domain DNS lookups can hang
+    // far past the connect timeout. On the shared Dispatchers.IO those hanging lookups
+    // starve the crawl's own IO — measured +462s on the finishing drain at hop-3. Run
+    // them on a dedicated, isolated daemon pool instead so the crawl's IO is untouched.
+    private val probeDispatcher =
+        Executors
+            .newFixedThreadPool(128) { r -> Thread(r, "relay-probe").apply { isDaemon = true } }
+            .asCoroutineDispatcher()
+
     /**
      * Cheap reachability pre-probe: a raw TCP connect (one round trip) with a tight
      * timeout. Returns false only when the port won't even accept a socket — a dead
@@ -139,7 +150,7 @@ object GrapeRankCommand {
      * so an odd URL is never culled on a parse quirk — let the WS decide.
      */
     private suspend fun tcpReachable(relay: NormalizedRelayUrl): Boolean =
-        withContext(Dispatchers.IO) {
+        withContext(probeDispatcher) {
             val hostPort = relayHostPort(relay) ?: return@withContext true
             try {
                 Socket().use { it.connect(InetSocketAddress(hostPort.first, hostPort.second), PROBE_TIMEOUT_MS) }